The present invention relates to a process for fabricating open-pore polymeric matrices using gas induced phase inversion. The present invention also relates to matrices prepared via this process of varying thickness and shape with open-pore morphologies, and pore sizes and densities ranging from, but not limited, about 10-500 xcexcm in diameter and about 0.1 to 0.3 g/cm3. In the case of biodegradable and biocompatible polymers, matrices prepared via this process with these characteristics are useful in tissue engineering/regeneration applications including, but not limited to, medical implant devices to promote bone healing, cartilage repair, cellular infiltration and tissue ingrowth. Constructs prepared from commodity, engineering, or other thermoplastic polymers may be useful for such applications as filtration and separation aids and porous supports.
A variety of techniques have been developed for fabricating open pore biodegradable polymer scaffolds for tissue engineering applications. The most common methods have been adapted from the polymer membrane industry and include such techniques as phase separation of polymer solutions induced either by thermal instability (Nam et al. J. Biomed. Mater. Res. 1999 47:8-18) or solvent (solubility) instabilities (Nunes, S. TRIP 1997 5(6):187-192; Cohen, J. et. al. Polymer Bulletin 1999 42:345-352), and particulate leaching (Freed et al. J. Biomed. Mater. Res. 193 27:11-23).
Porous polymer constructs produced using thermally induced phase separation (TIPS) have open-pore structures; however, the pore size is typically limited to approximately 10 to 20 xcexcm in diameter. Recently, porous devices with larger pore sizes and morphologies different from the typical ladder-like structures have been fabricated using TIPS as described in Nam et al. J. Biomed. Mater. Res. 1999 47:8-18, which is incorporated herein by reference. This was achieved by varying the quench depth and the coarsening times for morphology development.
Larger pores can be generated using solvent induced phase separation (SIPS), but asymmetric membranes with a dense skin layer are generally produced. Membranes fabricated by the SIPS technique are cast on a fabric support since the membranes themselves are only a few hundred microns thick. Typically, the membranes will have an asymmetric pore distribution and in some instances, finger like structures are produced. One of the disadvantages associated with SIPS processes is the cost of solvents and their disposal or purification.
A modified SIPS technique can also be applied in the fabrication of thicker and larger porous constructs of varying shape. However, development times required to produce these constructs are on the order of days to weeks. In the case of biodegradable polymers, long development times can be detrimental to the structural integrity of the construct as premature degradation can occur.
In order to achieve sufficient porosity and interconnectivity using leaching techniques, high poragen contents are required. This high poragen loading often leads to mechanically fragile matrices. In the case of low poragen loadings, complete removal of the poragen can be complicated by the presence of closed pores.
Recent efforts have been made in adapting microcellular foaming techniques to produce foams with open pore architecture. Typically, microcellular foaming processes yield closed-cell foams that have pores on the order of about 10 xcexcm and a pore density of  greater than 108 pores/cm3. In order to generate sufficient interconnectivity, biodegradable polymer compression molded with NaCl particles have been foamed using CO2 microcellular techniques (Harris et. al., Biomed. Mater. Res. 1998 42:396-402). Once foamed, the salt is leached out to create the interconnected pores.
Various methods for production of microcellular foams via the use of supercritical fluid antisolvents (or nonsolvents) have been described. See, for example, U.S. Pat. No. 5,066,684, U.S. Pat. No. 5,116,883, and U.S. Pat. No. 5,158,986. Use of supercritical fluid antisolvents has also been described for production of small particles and coatings of medicaments (U.S. Pat. No. 5,833,891), aerogels (U.S. Pat. No. 5,864,923), membranes (Matsuyama et al. J. of Appl. Polymer Sci. 1999 74:171-178), and in recrystallization of solid materials such as RDX, the explosive cyclotrimethylenetrinitramine (U.S. Pat. No. 5,360,478 and U.S. Pat. No. 5,389,263).
U.S. Pat. No. 5,422,377 discloses a process for producing thin microporous polymeric films for numerous uses including high energy physics targets, biomedical structures for tissue ingrowth, filters, low dielectric films for electronic devices, and asymmetric membranes. In this process, a polymer solution film, comprised of polymer dissolved in a non-volatile solvent, is subjected to a dense or pressurized gas that is not a solvent for the polymer but is soluble in the solvent. The dense gas diffuses into the film, and since the dense gas is soluble in the solvent of the film, but is a non-solvent for the polymer, phase separation occurs and two phases are formed. Simultaneous with the dense gas diffusing into the film is the diffusion of the solvent from the film out into the dense gas. Eventually little solvent remains in the film and the polymer will either glass or crystallize so that the phase separated morphology is locked in. When the pressure of the system is released, the dense gas leaves the film taking with it any remaining solvent while leaving behind a dry microporous polymer film with a cell morphology dependent upon the relationship that existed between the first and second phase.
Many studies have been performed to separate polymers from solution using a supercritical fluid (SCF; Gucke, T. et. al. U.S. Pat. No. 4946940; Mc Clellan A. et. al. Polym. Eng. Sci. 1985 25(17):1088-1092) but their emphasis has mainly been to purify the polymer. Kojima, J. et. al. (Macromol. 1999 32:1809-1815) have examined the early stages of spinodal decomposition in polymer solution under high pressure using light scattering techniques. Poly(vinylidene fluoride) membranes have been synthesized by using water vapor to induce nonsolvent phase separation (Matsuyama H. et. al. J. Appl. Polym. Sci. 1999 74:171-178).
Fabricating matrices which can be varied in thickness, shape and size, and contain interconnected pores of sufficient size, to be useful for the tissue engineering and medical device industries has been problematic.
An object of the present invention is to provide a method for producing open-pore polymeric matrices which comprises preparing a homogeneous polymer solution comprising one or more polymers and one or more solvents; treating the solution with a gas or supercritical fluid under selected conditions of pressure and temperature so that the gas or supercritical fluid is a nonsolvent or is sparingly soluble for the polymer; allowing the polymer solution to phase separate so that the polymer gels and precipitates from the solution, thus solidifying to form the matrix; if necessary, lowering the temperature of the solution to prevent the polymer from redissolving and help lock-in the pore morphology; and removing the residual solvent preferably via lyophilization. Typically the temperature is lowered near or below the freezing temperature of the solvent or near or below the plasticized glass transition temperature of the polymer/solvent/gas system so that further changes in morphology are minimized upon depressurization.
Another object of the present invention is to provide open-pore polymer matrices prepared via this process of gas induced phase inversion.
Yet another object of the present invention is to provide devices comprising open-pore polymer matrices prepared via this process. Devices which can be prepared include, but are not limited to, medical implant devices comprising open-pore biodegradable matrices and filtration/separation aids and porous supports from commodity, engineering and other thermoplastic polymers.
The present invention relates to a new process and open-pore polymer matrices prepared via this process. The process of the present invention offers several advantages over conventional methods for preparation of open-pore constructs by phase inversion techniques such as thermally induced phase separation (TIPS) and solvent induced phase separation (SIPS). Specifically, constructs produced via this process have open pore morphologies with pore sizes and densities ranging from, but not limited to, about 10-500 xcexcm in diameter and about 0.1 to 0.3 g/cm3, depending on the conditions used. The constructs can be fabricated into any shape and desired size and have open-pore surfaces and uniform pore morphologies that are easily controlled by varying process conditions to tailor specific applications. Replacing the traditional liquid non-solvent with a gas or supercritical fluid in the process of the present invention significantly reduces the amount of nonsolvent required in comparison to SIPS techniques, thereby reducing the costs associated with disposal or recycling. The development time of these constructs as compared to the modified SIPS technique for thicker devices is reduced from several weeks to a few days, thereby shortening the time the polymer is in contact with solvents or nonsolvents.
In this process, a polymer solution is prepared. The solution comprises one or more solvents and one or more polymers. In one embodiment of the invention, the polymer solution comprises approximately 10-25 wt % polymer such as poly(desaminotyrosyltyrosine[ethyl ester] carbonate) (p(DTE-co-0% DT carbonate)), or a copolymer such as poly(desaminotyrosyltyrosine [ethyl ester] carbonate-co-desaminotyrosyltyrosine carbonate) (poly(DTE-co-X%DT carbonate), where X % can range from approximately 1 to 100 percent with the appropriate corresponding decrease in DTE composition) in a solvent such as 1,4-dioxane or N-methyl-2-pyrrolidone or mixtures of 1,4-dioxane or N-methyl-2-pyrrolidone with water or tetrahydrofuran (THF). The polymer solution is then poured into a container or mold that is open to a gas or supercritical fluid. Suitable containers include, but are not limited to petri dishes, aluminum pans, glass vials, plastic containers and gas permeable membranes, such as dialysis membranes. The solution is then treated with a gas such as carbon dioxide (CO2) or a supercritical fluid (SCF) under pressure and temperature conditions in which the gas or supercritical fluid is a nonsolvent or is sparingly soluble for the polymer.
For example, a 10-25 wt % polymer solution such as p(DTE-co-0% DT carbonate) or poly(DTE-co-X % DT carbonate) dissolved in 1,4-dioxane is preferably treated with CO2 at room temperature for a prescribed time period sufficient for the gas to diffuse into the polymer solution, resulting in a phase separated system. During phase separation the polymer in the solution gels and precipitates, thereby solidifying to form the matrix. However, the polymer-rich phase at this point is still imbued with solvent and if the pressure is released, the polymer will redissolve. Hence, after the polymer solution has been treated with CO2, the system is cooled to about 0xc2x0 C. for a prescribed time period to fix or lock-in the pore morphology and minimize further changes to the matrix during depressurization. The CO2 is then released, and the samples are removed from the pressure vessel. The samples are kept cold to prevent the solvent from melting and/or dissolving the polymer. The remaining solvent is then removed from the porous devices preferably via lyophilization.
As will be obvious to those of skill in the art upon this disclosure, however, other polymers at various concentration ranges, other solvents and gases or supercritical fluids, and other conditions including saturation temperatures, times and pressures, and cooling times and temperatures, than those exemplified by this embodiment can be used in this process. Additional exemplary polymers which can be used in this process include, but are not limited to, polyolefins such as polyethylene and polypropylene, polymethacrylates such as polymer(methyl acrylate) and poly(hydroxy ethyl methacrylate), polycarbonates such as poly(bis-phenol A carbonate), styrenic polymers such as polystyrene, block copolymers such as styrene-ethylene-butylene-styrene, vinyl based polymers such as poly(vinyl chloride), polysulfones such as polysulfone and poly(ether sulfone), and polyesters such as poly(ethylene terephthalate). Further, polymers used in these matrices can be modified by attachment of molecules which enhance the utility of the matrix. For example, in one embodiment, wherein the matrix is used as a medical implant, the polymer can be modified to include a molecule which enhances cell attachment or cell growth. In another embodiment, wherein the matrix is used in filtration, the material can be made more hydrophilic or hydrophobic depending upon the application and need for easy wetting of the porous construct. In a preferred embodiment, the polymer comprises poly(DTE-co-1% DT carbonate), poly(DTE-co-X % DT carbonate) where X ranges from about 1 to about 100%, poly(DTE-co-X % DT carbonate) where X ranges from about 1 to about 100% onto which an RGD containing peptide or mimetic thereof is attached, poly(methyl methacrylate), polystyrene, poly(glycolic acid), poly(lactic acid), or copolymers of glycolic and lactic acid. However, it is believed that any polymer/solvent/gas or SCF system can be used in this process provided that the starting conditions and final conditions fall within the ranges required for spinodal decomposition. Further, homogeneous polymer solution phase separates if the two phases are more thermodynamically stable as two separate phases. As demixing occurs, polymer-rich and solvent-rich phases are created. If the homogenous polymer solution is taken from one phase state (above the binodal line) to a condition between the binodal and spinodal line, the polymer solution phase separates by a nucleation and growth type mechanism. This results in formation of individual droplets and a closed pore-type morphology. If the system is taken from one phase state to a point below the spinodal line, phase separation occurs via a spinodal mechanism and leads to the formation of an interconnected, continuous type morphology. Further, a cooling step is not always required. For example, if the phase separated system is stable upon depressurization or if the saturation temperature is sufficiently low, cooling is unnecessary.
Thus, various polymers (degradable and non-degradable), polymer concentrations, solvents and gases and/or supercritical fluids can be selected and used in the process of the present invention. Selection of the polymer is based upon its solubility in the selected solvent and its insolubility in the selected gas/SCF under the conditions used. Concentration of the polymer is selected in accordance with the desired final density of the matrix. When higher concentrations of polymer are used or a different polymer/solvent system is used, it may be necessary to raise the saturation temperature at which the polymer solution is treated with the gas or supercritical fluid so that the polymer solution is homogeneous and does not precipitate from solution before contact with the gas or SCF. Selection of the solvent is based upon its ability to dissolve the selected polymer. Preferred solvents have a freezing point between approximately xe2x88x9240xc2x0 C. and 15xc2x0 C. so that the pore morphology of the matrix can be locked in and further changes minimized upon depressurization by lowering the temperature of the system below or near the freezing point of the solvent, or below or near the plasticized glass transition temperature of the polymer/solvent/gas system. When polymers are swollen with solvent or contain dissolved gases, transition temperatures may be depressed (Chiou J. et. al. J. Appl. Polym. Sci. 1985 30:2633-2642), thus allowing them to flow at much reduced temperatures than would otherwise be expected. The glass transition temperature is the temperature at which an amorphous or partially amorphous polymer crosses from a rigid glassy state to one that is capable of flowing. Preferred solvents also have a moderate boiling point so that the solvent can be easily removed to fix the pore morphology and minimize further changes to the matrix. It should be noted that a cooling step is not always required to fix the pore morphology and minimize further changes to the matrix. For example, if the phase separated system is stable upon depressurization or if the saturation temperature is sufficiently low cooling is not required.
Using combinations of solvents is another technique of changing the morphology of the constructs. The addition of a nonsolvent to the polymer will effect the kinetics and pathways of phase separation, leading to different structures in the final device. Similarly, combinations of gases and blends of polymer can also be employed.
Selected gases and supercritical fluids used in this method must have relatively low solubilities in the selected polymer under the conditions of temperature and pressure to be used. A preferred gas is CO2. However, supercritical fluids (SCF) such as supercritical CO2 can also be used. The solvent power of CO2 is dependent on its density. Thus, using CO2 in the SCF state will have effects on the phase inversion process depending on the temperature and pressure of the system, as SCF CO2 is a tunable solvent. The selected gas plays a major role in pore morphology development.
The most commonly used supercritical fluid (SCF) solvents exist as liquids or gases at ambient pressures and temperatures. When compressed and heated to conditions above their critical points, these compounds become supercritical and have densities and transport properties that are intermediate between their liquid and gaseous states. Once a compound is at a temperature and pressure greater than its critical point, it exists as one phase no matter how much it is compressed. The critical point for CO2 is 31.1xc2x0 C. and 7.37 MPa. The solvent capacity of SCF CO2 is density dependent and, thus, can be controlled by changing the temperature and pressure to vary the solvent power between that of a gas and liquid. This property is what makes a SCF a tunable solvent, a characteristic that is not available with traditional liquid solvents. The critical properties of a series of compounds commonly used as SCF solvents are listed in Table 1.
A series of samples were prepared in accordance with the process of the present invention. For these samples, a 20 wt % poly(DTE-co-0% DT carbonate) solution in 90:10 dioxane:water was treated with CO2. When the same solution was cooled to 0xc2x0 C. but not exposed to CO2, it did not freeze or solidify. However, when cooled to about xe2x88x9215xc2x0 C., the polymer and solvent of the untreated solution did phase separate by thermally induced phase separation (TIPS). The resulting morphology of this device was significantly different from the device prepared in accordance with the present invention. The device prepared in accordance with the process of the present invention has large rounded pores, on the order of about 100 to 150 xcexcm, which are interconnected. The pore walls are smooth and there are several interconnecting holes in the walls. In contrast, the device prepared by simply cooling the polymer solution to xe2x88x9215xc2x0 C. has pores on the order of about 10-15 xcexcm. These pores are arranged such that they resemble a series of parallel ladders-typical of the structure formed by TIPS.
Examination of various matrices prepared via the process of the present invention revealed that 20 wt % poly(DTE-co-0% DT carbonate) in 90:10 dioxane water produced a very homogeneous matrix. Strong visual evidence of pore interconnectivity was also observed by scanning electron microscopy (SEM). Pore interconnectivity was confirmed by mercury porosimetry analyses.
The sample described in the preceding paragraphs was prepared inside of a pressure vessel, and it was uncertain as to when or how phase inversion actually occurred.
Accordingly, a critical point drying apparatus (CPDA) was modified such that the phase inversion process of the present invention could be observed. In addition, the CPDA permits screening of potential candidates for this process and determination of the operating window for phase inversion. Any pressure vessel equipped with a viewing window would suffice for this purpose.
The CPDA used in these experiments comprises a pressure chamber along with a series of valves, a glass window, pressure and temperature gauges, and a thermostat jacket. The diameter of the pressure vessel is 1 inch and the length of the barrel is 3 inches. The pressure rating is 1800 psi. Polymer solution or solvent/solvent mixtures were poured into glass vials (8 mm inner diameterxc3x9715 mm) filled approximately ⅓ to ⅔ full. Graduated stickers were placed on the vials to aid in monitoring changes in the system during the phase inversion process. A maximum of three sample vials were attached at the end of a glass microscope slide and inserted into the CPDA. The CO2 pressure applied on the system was slowly raised and the phase inversion process monitored.
In general, when the solutions were brought into contact with CO2, the liquid-gas interface became turbulent. As CO2 dissolved into the solutions, their volumes expanded and the system pressure decreased. Eventually, a CO2 rich front developed and moved down through the solution. When the dissolved CO2 concentration surpassed a critical level, the polymer solutions began to phase separate. The surface in contact with the CO2 precipitated from solution first. A small increase in the construct height is observed due to the sorption of CO2 into the polymer solution before the polymer precipitates from solution. Once the first solid layer of polymer formed, the relative change in height was negligible. As the process continued, a layer of solvent was deposited on top of the phase separated polymer. The fact that the top surface is porous allows the flow of solvent out as more CO2 is dissolved into the solution. Thus, the process of the present invention is different from the process disclosed in U.S. Pat. No. 5,422,377 as the porous construct is not dry after depressurization. Instead, the residual solvent is removed by lyophilization. In the process of the instant invention, the amount of solvent that has to be lyophilized can be reduced by adding more polymer solution such that it occupies most of the volume of the container. As the CO2 sorbs into the polymer, the aliquot of solvent that is displaced by the sorption of CO2 would spill over the top. There would, however, still be some solvent remaining in the pores and in the bulk of the material.
The CPDA can also be used to observe methods for controlling the pore size in this process through varying of the polymer concentration, changing the solvent or adding a cosolvent/nonsolvent, varying the pressure, changing the temperature, using different gases and/or supercritical fluids or a mixture thereof, and pre-conditioning the polymer solution so that the solution expands but the polymer does not precipitate from solution followed by an increase in the pressure to induce phase inversion.
Some of the samples were made from different lots of polymer. Though the chemical composition is the same, their molecular weights are different. This may lead to some of the variation in pore size for matrices made using similar conditions as the solvent viscosities may vary resulting in slightly different CO2 diffusion rates, thus influencing the kinetics of phase separation.
Characteristics of the porous polymer matrices prepared via the process of the present invention are within the range considered to render them useful as medical implants for bone and cartilage repair. More specifically, biodegradable devices can be prepared via the process of the present invention with pore sizes ranging from about 10 to about 500 xcexcm, more preferably about 100 to about 250 xcexcM, in diameter and open porosities in the range of about 90% to about 100%, more preferably about 100%.
Further, porous constructs can be prepared via the process of the present invention in approximately 1 to 3 days depending on the polymer/solvent/gas system and thickness of the device.
Thus, an advantage of the process of the present invention is that it minimizes the time the polymer is in contact with solvents. This new process is also adaptable to other polymers including, but not limited to PS and PMMA, as well as other solvents and gases. Temperature and pressure conditions for processing matrices from various polymers, solvents and gases are selected in accordance with the phase diagram of the system. As demonstrated herein, this process also allows for variations in pore morphology of matrices depending upon selected uses via changes in polymer concentration, solvent ratio and pressure without affecting pore size.
This technique can also be used to coat other biomedical implants, such as metal implants which require close bone apposition. The fact that the surface of the constructs prepared via the method of the present invention are open pore allows for cell ingrowth into the coating/matrix which provides a temporary scaffold for bone to regenerate, thus allowing for a more spatially defined growth.
Matrices prepared in accordance with this process from commodity, engineering and other thermoplastic polymers are also useful as filtration/separation aids and porous supports.